The invention relates to a method for preparing comb-branched polymers. More particularly, the invention relates to a continuous polymerization process for making a copolymer of a polyether macromonomer and an acrylic monomer. The copolymers are valuable water reducing agents for cement.
Water reducing agents reduce the amount of water needed in cement admixtures, while maintaining good processing ability and consistency. Lignin sulfonates and naphthalene sulfonate-formaldehyde condensates have long been used as water reducing agents. These conventional water reducing agents are readily available and relatively inexpensive. However, they are used in high doses.
In contrast, newly developed polymeric water reducing agents offer high performance but are more expensive to make. U.S. Pat. No 4,814,014, for example, teaches to graft ethylenically unsaturated monomers onto a polyether. The graft copolymer is used at a low dosage. Unfortunately, it is contaminated with a large portion of non-grafted polyether and ethylenic homopolymer. Because these non-grafted polymers do not function as water reducing agents, they reduce the effectiveness of the product.
Comb-branched copolymers of acrylic acid and polyether macromonomers have been used as high performance water reducing agents (see U.S. Pat. No. 5,834,576). The comb-branched copolymers have more uniform structures compared to the graft polymers of U.S. Pat. No. 4,814,014. Consequently, they have higher water reducing ability. An added advantage of these copolymers is the improved ability to maintain xe2x80x9cslump.xe2x80x9d Slump retention is the workable time after the cement admixture is mixed. Commonly used polyether macromonomers include acrylates, methacrylates, and allyl ethers of polyether.
Methods for preparing comb-branched copolymers of carboxylic monomers and polyether macromonomers are known and relatively simple. In general, free radically polymerizing a polyether macromonomer with a carboxylic monomer forms a comb-branched copolymer. While the related literature briefly mentions batch, semi-batch, and continuous processes (see U.S. Pat. No. 5,834,576, and copending application Ser. No. 09/074,673), no one has suggested that a continuous process would offer comb-branched copolymers that perform better in cement compositions. Specific teachings about how to conduct a continuous process for making comb-branched copolymers are not available. U.S. Pat. No. 5,834,576, for example, only teaches details of a batch process.
The invention is a continuous process for making a comb-branched copolymer of an acrylic monomer and a polyether macromonomer. The process comprises: (a) forming a monomer stream, an initiator stream, and an optional chain transfer agent stream; (b) polymerizing the streams in a reaction zone at a temperature within the range of about xe2x88x9220xc2x0 C. to about 150xc2x0 C.; and (c) withdrawing a polymer stream from the reaction zone.
The invention also includes a multiple-zone process that comprises: (a) forming a monomer stream, an initiator stream, and an optional chain transfer agent stream; (b) polymerizing the streams in a first reaction zone at a temperature within the range of about xe2x88x9220xc2x0 C. to about 150xc2x0 C.; (c) transferring a first polymer stream from the first reaction zone to a second reaction zone wherein the polymerization continues; and (d) withdrawing a second polymer stream from the second reaction zone. The multiple-zone process enhances monomer conversion and process efficiency.
We surprisingly found that the comb-branched copolymers made by the process of the invention perform significantly better as water reducing agent in cement compared with polymers made by a batch process. They offer higher slump and flow.